1. Field of the Invention
The present invention relates to an image forming apparatus such as a digital copying machine and a laser printer and an image write start position adjusting method for the same, and in particular to an image forming apparatus that transfers and fixes an image onto a transfer material, and is capable of two-sided image formation or multiple image formation on the transfer material, and an image write position adjusting method for the same.
2. Description of the Related Art
Conventionally, there has been known an image forming apparatus, such as a digital copying machine, which deflects a light beam using a polygon mirror to scan light across an image carrier to form a latent image pattern by means of a light scanning device, and thereafter carries out by developing the latent image and transferring the developed image onto a transfer material, and then fixing the image using heat or the like. Adjustment of image magnification in this kind of image forming apparatus is normally carried out when the apparatus is assembled. The image magnification can be adjusted by measuring the time period taken by a light beam, which is continuously emitted and is deflected by the polygon mirror to scan the image carrier, to pass two sensors provided a predetermined distance apart in the scanning direction at positions corresponding to the surface of the image carrier and adjusting the position of a reflective mirror disposed midway on the optical path, according to the measured time period. By this adjustment, the length of the optical path from the polygon mirror to the surface of the image carrier is changed, so that the desired image magnification can be obtained.
In the image forming apparatus adjusted using the above method, the latent image formed on the image carrier is developed by a developing device to form a toner image which is transferred onto the transfer material. The toner image which has been transferred on the transfer material is fixed by a fixing device by applying heat thereto under pressure and the transfer material is discharged from the image forming apparatus, or alternatively the transfer material is conveyed to a transfer section once more for two-sided or multiple image formation. The application of heat to the transfer material here results in the transfer material slightly expanding or contracting. That is, when an image is copied at 100% magnification or equimultiplication, the resulting image is minutely expanded or reduced.
The amount of expansion/contraction of the transfer material is around 0.5% at most in both a conveying direction (hereinafter referred to as the “sub scanning direction”) and a direction perpendicular to the conveying direction (hereinafter referred to as the “main scanning direction”), and is normally not problematic. However, in cases such as when a blueprint is outputted or when the output sheet is used as a paper pattern, such expansion/contraction cannot be tolerated. One way to cope with this would be a method of adjusting in advance the magnification of the image forming apparatus so as to compensate for the expansion/contraction of the transfer material, but the expansion/contraction differs depending on the fixing temperature, the transfer material type, and the orientation of fibers of the transfer material, so that it is difficult to universally determine the amount of adjustment of the magnification.
For this reason, to adjust the image magnification in the main scanning direction on the transfer material, a means that adjusts the magnification in the main scanning direction by modulating an image clock and/or adjusts the magnification in the sub scanning direction or the magnification in the main and sub scanning directions by modulating the rotational speed of the polygon mirror and modulating the image clock has been. proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H05-281487).
Further, a device that, when forming images on both sides (surfaces) of the transfer material, changes the size of a second surface image by adjusting the rotational speeds of the reading apparatus and the image carrier relative to the rotational speeds for a first surface image to thereby compensate for the expansion/contraction of the transfer material after fixing has been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H07-72674, for example).
However, there has been the following problem with the above conventional image forming apparatus. That is, although there are no problems when image magnification is carried out using the reading apparatus in the case where the image forming apparatus is used as a copying machine, in the case where the image forming apparatus is used as a printer, there is the problem that the image forming apparatus cannot appropriately handle image information sent from a personal computer (PC) or the like connected to the image forming apparatus.
When using the method of adjusting the main scanning magnification by modulating the image clock, it is possible to appropriately deal with image information when the image forming apparatus is used as a printer and a proper, full-size (100% magnification) image can be obtained. Also, by further modulating the image clock in accordance with the expansion/contraction of the transfer material during two-sided or multiple image formation, it is possible to obtain the correct sizes for the first surface image and the second-side image.
However, an image forming apparatus equipped with a light scanning device that uses a polygon mirror as a deflector is generally provided with a horizontal synchronization detecting device that uses a horizontal synchronization signal (beam detect (BD) signal) as a reference for controlling a latent image write start position on the surface of the image carrier and obtains the BD signal before the write start position is reached. The horizontal synchronization detecting device has a BD signal detecting section or the like and detects the BD signal by receiving part of a light beam that has been deflected and reflected by the deflector, and is provided inside the light scanning device or the image forming apparatus. The horizontal synchronization detecting device is normally disposed at such a location that the detection position for the BD signal lies outside the width (i.e., length in the main scanning direction) of the widest transfer material that can be conveyed.
In order to determine an image position on the transfer material, a transfer material position determining device that determines the position of one end edge (normally on the scanning start side of the light scanning device) of the transfer material in the main scanning direction is normally provided inside the image forming apparatus. In an image forming apparatus in which two-sided image formation is carried out by inverting the transfer material according to a switchback method or multiple image formation is carried out without inverting the transfer material, image formation is carried out after the image position has been determined for an end edge on the same side of the first surface and the second surface. This can result in that when two-sided image formation or multiple image formation is carried out, image displacement occurs between the first surface and the second surface of the transfer material in the main scanning direction.
This will now be described in more detail with reference to FIGS. 9A to 9C. As one example, it is assumed that a synchronization detection device 155 is disposed at a position 10 mm outside a transfer material positioning device (a main scanning reference end edge of the transfer material) F. It is further assumed that the maximum width of the transfer material that can be conveyed is 305 mm and the image write start position is 2.5 mm from the end edge of the transfer material. After a number of image clock pulses corresponding to 12.5 mm have been counted following detection of the BD signal, scanning with light is carried out to form an image on the surface of the image carrier based on image information. After a 300 mm-image has been formed, the light is temporarily extinguished, necessary control, such as APC (automatic power control), is carried out, the deflection surface of the polygon mirror is changed, and the same scanning is repeated. As a result, as shown in FIG. 9A, a toner image that is 300 mm wide in the main scanning direction with a margin of 2.5 mm being left on the write start side of the first surface of the transfer material is formed according to the image formation process. After image fixing, the transfer material passes a two-sided or multiple conveying path and image formation on the second surface is carried out. After image formation is carried out on the first surface, the application of heat to the transfer material for fixing causes a 0.5% contraction of the transfer material in the main scanning direction. This result in that as shown in FIG. 9B, the first surface image has a margin on the write start side of 2.4875 mm (=2.5 mm×0.995) and is 298.5 mm(=300 mm×0.995) wide.
Next, when image formation is carried out for the second surface, if the rotational speed of the polygon mirror is unchanged, the frequency of the image clock that is modulated according to the expansion/contraction ratio becomes 1/0.995 of the frequency used during image formation on the first surface.
As shown in FIG. 9C, if scanning with light based on image information is started after the same number of pulses have been counted as during image formation on the first surface, the write start position of the latent image will be 12.4375 mm(=12.5 mm×0.995) from the synchronization detection device 155, and the latent image will be 298.5 mm(=300 mm×0.995) wide. Since the position of the transfer material positioning device F does not change, that is, since the distance from the synchronization detection device 155 to the transfer material positioning device F remains at 10 mm, the margin on the image write start side of the transfer material is 2.4375 mm. This results in that there is a difference of 50 μm in the sizes of the margins on the write start position side following image fixing (see FIG. 9B) on the first surface, leading to a displacement in the positions in the main scanning direction between the images on the first and second surfaces.
Next, an example of prominent image displacement will be described with reference to FIGS. 10A to 10C. In an image forming apparatus in which the center of the transfer material in the main scanning direction always passes near the center of an image forming section, the transfer material positioning device F is normally movable. In this image forming apparatus, when an A4-size transfer material is conveyed in a direction R (the width in the main scanning direction is 210 mm), the transfer material positioning device F moves to a position 57.5 mm from the synchronization detection device 155. If the margin on the first surface is set at 2.5 mm as in the above described example, as shown in FIG. 10A, scanning with light based on image information is started after a number of pulses corresponding to 60 mm have been counted following detection of the BD signal. The width of the image in the main scanning direction is set at 205 mm.
If the transfer material contracts by 0.5% due to image fixing, as shown in FIG. 10B, the margin is 2.4875 mm and the image is 203.975 mm wide. FIG. 10C shows that due to image clock modulation, the latent image write start position for scanning with light for the second surface is 59.7 mm from the synchronization detection device 155, the margin is 2.2 mm, and the image is 203.975 mm wide. Accordingly, the images on the first and second surfaces are displaced by almost 300 μm in the main scanning direction.
In recent years, there has been increasing demand for image forming apparatuses that carry out a latent image forming process as POD (print on demand) devices. Output sheets are often cut and folded as post processing, so that image displacements when carrying out two-sided or multiple image formation pose a big problem.